Aircraft wheels may include separate drive key members that are attached by bolts to a wheel. The drive key members are engaged by rotors of an aircraft brake. During braking, the drive keys must withstand substantial braking torque as the brake rotors engage the brake stators. The drive key members may be made from hardened and tempered steel to provide a high tensile strength, or a nickel alloy such as Inconel. Inconel drive keys have an inherent corrosion resistance, but are significantly (two to three times) more expensive than steel drive keys. Steel drive keys, while only a fraction of the cost of Inconel drive keys, are subject to corrosion which can result in cracking and failure of the steel.
In some brake systems, steel drive key members are coated with a chromium plating which provides an adequate degree of protection in certain environments. The cost of chromium plating is also low enough that it provides a cost advantage over alloys such as Inconel. However, when carbon-carbon composite brake discs are used in such systems, carbon dust from the brake discs reacts with the chromium plated steel drive keys due to the inherent microcracking in the chromium plating and impairs the effectiveness of the coating. Once the coating is breached, water, cleaning agents and de-icers, for example, may reach the steel member and cause corrosion. The subsequent corrosion plus the high stresses to which the drive keys are subjected results in stress corrosion cracking and may lead to a failure of the drive key.
One way of addressing this problem is by coating the drive key with an electroless nickel plating that forms a barrier over the steel and tungsten carbide coating on the side rails which insulates the steel from the carbon dust. This method may be more expensive than using chrome, but still provides a cost advantage over alloys such as Inconel. Such a method is described, for example, in U.S. Pat. No. 5,931,269 entitled “Aircraft Wheel Part With Corrosion and Wear Resistance,” the disclosure of which is hereby incorporated by reference.
It has been found, however, that the electroless nickel coating can be damaged by the stresses applied to the drive keys during use and/or by the scraping of the rotor mounting elements against the drive keys when the rotors are removed for repair or replacement. Once the coating is breached, carbon dust from carbon-carbon brakes reaches the steel beneath the coating and begins to corrode the steel. Repairing electroless nickel is also environmentally unfriendly, time consuming and expensive. Thus, while electroless nickel provides benefits over chromium coatings, it still provides fewer benefits than expensive alloys such as Inconel. It is therefore desirable to provide a steel member, suitable for use as an aircraft wheel rotor drive key, that exhibits corrosion resistance similar to that exhibited by expensive alloys but at a significantly lower cost.